# Django 2.0.dev20170216164632 documentation

# Source code for django.middleware.http

    from django.utils.cache import (
        cc_delim_re, get_conditional_response, set_response_etag,
    )
    from django.utils.deprecation import MiddlewareMixin
    from django.utils.http import parse_http_date_safe

    class ConditionalGetMiddleware(MiddlewareMixin):
        """
        Handles conditional GET operations. If the response has an ETag or
        Last-Modified header, and the request has If-None-Match or
        If-Modified-Since, the response is replaced by an HttpNotModified. An ETag
        header is added if needed.
        """
        def process_response(self, request, response):
            # It's too late to prevent an unsafe request with a 412 response, and
            # for a HEAD request, the response body is always empty so computing
            # an accurate ETag isn't possible.
            if request.method != 'GET':
                return response

            if self.needs_etag(response) and not response.has_header('ETag'):
                set_response_etag(response)

            etag = response.get('ETag')
            last_modified = response.get('Last-Modified')
            if last_modified:
                last_modified = parse_http_date_safe(last_modified)

            if etag or last_modified:
                return get_conditional_response(
                    request,
                    etag=etag,
                    last_modified=last_modified,
                    response=response,
                )

            return response

        def needs_etag(self, response):
            """
            Return True if an ETag header should be added to response.
            """
            cache_control_headers = cc_delim_re.split(response.get('Cache-Control', ''))
            return all(header.lower() != 'no-store' for header in cache_control_headers)